/*-
 * Copyright (c) 1991 The Regents of the University of California.
 * All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * Edward Sze-Tyan Wang.
 *
 * %sccs.include.redist.c%
 */

#ifndef lint
static char sccsid[] = "@(#)forward.c	5.4 (Berkeley) 02/12/92";
#endif /* not lint */

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "extern.h"

static void rlines __P((FILE *, long, struct stat *));

/*
 * forward -- display the file, from an offset, forward.
 *
 * There are eight separate cases for this -- regular and non-regular
 * files, by bytes or lines and from the beginning or end of the file.
 *
 * FBYTES	byte offset from the beginning of the file
 *	REG	seek
 *	NOREG	read, counting bytes
 *
 * FLINES	line offset from the beginning of the file
 *	REG	read, counting lines
 *	NOREG	read, counting lines
 *
 * RBYTES	byte offset from the end of the file
 *	REG	seek
 *	NOREG	cyclically read characters into a wrap-around buffer
 *
 * RLINES
 *	REG	mmap the file and step back until reach the correct offset.
 *	NOREG	cyclically read lines into a wrap-around array of buffers
 */
void
forward(fp, style, off, sbp)
	FILE *fp;
	enum STYLE style;
	long off;
	struct stat *sbp;
{
	register int ch;
	struct timeval second;
	fd_set zero;

	switch(style) {
	case FBYTES:
		if (off == 0)
			break;
		if (S_ISREG(sbp->st_mode)) {
			if (sbp->st_size < off)
				off = sbp->st_size;
			if (fseek(fp, off, SEEK_SET) == -1)
				ierr();
		} else while (off--)
			if ((ch = getc(fp)) == EOF) {
				if (ferror(fp))
					ierr();
					break;
				}
		break;
	case FLINES:
		if (off == 0)
			break;
		for (;;) {
			if ((ch = getc(fp)) == EOF) {
				if (ferror(fp))
					ierr();
				break;
			}
			if (ch == '\n' && !--off)
				break;
		}
		break;
	case RBYTES:
		if (S_ISREG(sbp->st_mode)) {
			if (sbp->st_size >= off &&
			    fseek(fp, -off, SEEK_END) == -1)
				ierr();
		} else if (off == 0) {
			while (getc(fp) != EOF);
			if (ferror(fp))
				ierr();
		} else
			bytes(fp, off);
		break;
	case RLINES:
		if (S_ISREG(sbp->st_mode))
			if (!off) {
				if (fseek(fp, 0L, SEEK_END) == -1)
					ierr();
			} else
				rlines(fp, off, sbp);
		else if (off == 0) {
			while (getc(fp) != EOF);
			if (ferror(fp))
				ierr();
		} else
			lines(fp, off);
		break;
	}

	/*
	 * We pause for one second after displaying any data that has
	 * accumulated since we read the file.
	 */
	if (fflag) {
		FD_ZERO(&zero);
		second.tv_sec = 1;
		second.tv_usec = 0;
	}

	for (;;) {
		while ((ch = getc(fp)) != EOF)
			if (putchar(ch) == EOF)
				oerr();
		if (ferror(fp))
			ierr();
		(void)fflush(stdout);
		if (!fflag)
			break;
		/* Sleep(3) is eight system calls.  Do it fast. */
		if (select(0, &zero, &zero, &zero, &second) == -1)
			err("select: %s", strerror(errno));
		clearerr(fp);
	}
}

/*
 * rlines -- display the last offset lines of the file.
 */
static void
rlines(fp, off, sbp)
	FILE *fp;
	long off;
	struct stat *sbp;
{
	register off_t size;
	register char *p;

	if (!(size = sbp->st_size))
		return;

	if ((p = mmap(NULL,
	    size, PROT_READ, MAP_FILE, fileno(fp), (off_t)0)) == (caddr_t)-1)
		err("%s", strerror(errno));

	/* Last char is special, ignore whether newline or not. */
	for (p += size - 1; --size;)
		if (*--p == '\n' && !--off) {
			++p;
			break;
		}

	/* Set the file pointer to reflect the length displayed. */
	size = sbp->st_size - size;
	WR(p, size);
	if (fseek(fp, sbp->st_size, SEEK_SET) == -1)
		ierr();
}